Technological Field
The present invention relates to an image forming method.
Description of Related Art
An inkjet recording method is employed in various fields of printing because images can be simply and inexpensively formed by this method. As one type of inkjet inks, an ink cured through irradiation with actinic radiation is known. Such an actinic radiation-curable ink usually contains an actinic radiation-curable compound and a photopolymerization initiator. When an image is formed by using the actinic radiation-curable ink, a droplet of the ink is caused to land on a surface of a recording medium, and the droplet thus caused to land is irradiated with actinic radiation. Thus, a cured product (a cured film) of the droplet is formed on the surface of the recording medium, so as to form a desired image. When an image forming method using the actinic radiation-curable ink is employed, an image having high adhesiveness can be formed regardless of the water absorbency of a recording medium, and hence, the image forming method is regarded promising.
As one type of the actinic radiation-curable ink, an ink that contains a gelling agent to reversibly undergo sol-gel phase transition through temperature change (hereinafter sometimes simply referred to as a “gel ink”) has been developed. A gel ink is in a sol state when heated and is in a gel state at a low temperature. Such a gel ink is in a sol state when discharged from a nozzle of an inkjet head, and is cooled to undergo gelation when it lands on a surface of a recording medium. Then, the gel ink thus pinned on the surface of the recording medium is irradiated with actinic radiation to cure the gel ink, and thus, an image is formed.
Here, it is known that an oxygen concentration is adjusted in irradiating a droplet of a gel ink with the actinic radiation (see, for example, Japanese Patent Application Laid-Open No. 2010-17710). According to this literature, a gel ink is cured while the oxygen concentration is adjusted, so as to adjust glossiness of an image to be formed.
On the other hand, it is also known that the oxygen concentration is adjusted also in curing an actinic radiation-curable ink other than the gel ink (see, for example, Japanese Patent Application Laid-Open No. 2014-159114). Here, a radical polymerizable compound and a cationic polymerizable compound are used as an actinic radiation-curable compound in many of actinic radiation-curable inks, and it is known that polymerization of a radical polymerizable compound is easily inhibited by oxygen. Therefore, in this literature, the inhibition of the polymerization by oxygen is suppressed by reducing an amount of oxygen present around the ink (an oxygen concentration or an oxygen partial pressure) at the time of the irradiation with the actinic radiation, and thus, hardness reduction of a cured film, degradation of abrasion resistance, occurrence of blocking and the like are suppressed.